struct ListNode
{
    int val;
    ListNode *next;
    ListNode() : val(0), next(nullptr) {}
    ListNode(int x) : val(x), next(nullptr) {}
    ListNode(int x, ListNode *next) : val(x), next(next) {}
};


class Solution {
public:
    ListNode* reverseList(ListNode* head) {
        if (!head || !head->next) return head;
        ListNode* reverseHead = reverseList(head->next);    // ak -> ak+1 <- ak+2
        head->next->next = head;
        head->next = nullptr;
        return reverseHead;

    }
};